1. Field of the Invention
This invention relates to a sintered body composed of .alpha.'-silicon nitride and .beta.'-silicon nitride, and having high strength and high resistance to wear, such as erosive wear, and a process for making the same.
2. Description of the Prior Art
A sintered body of silicon nitride (Si.sub.3 N.sub.4) has come to be used for making gas turbine parts, heat exchangers, bearings, etc. because of its high strength, thermal shock resistance, and corrosion resistance. A rapidly rotating body, such as a gas turbine blade, is, however, worn by, for example, dust floating in the air. For example, a plant using coal is seriously worn by the dust of coal. A sintered body of Si.sub.3 N.sub.4 has also come to be used for making tools for cutting heat-resisting alloys, or other tools required to have a high level of wear resistance. A known sintered body consisting mainly of .beta.-Si.sub.3 N.sub.4 is, however, too low in hardness to exhibit a high degree of wear resistance. It is known that a high degree of hardness is a highly desirable factor for any material to be able to exhibit excellent resistance to erosion and other wear.
Silicon nitrides of the type in which various elements form solid solutions, which are generally known as sialons, have come to draw attention because of their high hardness. One example is .alpha.'-Si.sub.3 N.sub.4 (or .alpha.-sialon) which occurs when Al replaces Si, while O replaces N, in the crystal lattice of .alpha.-Si.sub.3 N.sub.4, and another element, such as Li, Mg, Ca or Y, occupies the interstices of the lattice. It is represented by the general formula: EQU M.sub.x (Si, Al).sub.12 (O, N).sub.16
where M is at least one of such elements as Li, Mg, Ca and Y, and 0&lt;x.ltoreq.2. Another example is .beta.'-Si.sub.3 N.sub.4 (or .beta.-sialon) which is formed by the replacement of Si by Al, and of N by O, in the crystal lattice of .beta.-Si.sub.3 N.sub.4, and which is represented by the general formula: EQU Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z
where 0&lt;z.ltoreq.4.2.
Although .alpha.'-Si.sub.3 N.sub.4 is superior to .beta.'-Si.sub.3 N.sub.4 in hardness and wear resistance, it is inferior in toughness and strength. Therefore, a sintered body composed of .alpha.'-Si.sub.3 N.sub.4, .beta.'-Si.sub.3 N.sub.4 and a bonding phase has been developed to provide a material which is excellent in both wear resistance and strength, as disclosed in Japanese Patent Publications Nos. 8074/1988 and 8075/1988. This material is, however, still unsatisfactory in strength, as it contains a relatively large amount of bonding phase.